The present invention relates to a negative type photosensitive electrodepositing resin composition, an electrodepositing bath using the same, and a process for producing a resist pattern using the same.
In the production of a printed circuit board, a layer of a photocurable resin composition is firstly formed on a substrate, then an actinic light is imagewisely projected, and the uncured area is removed by development to form a resist pattern. In this process, a variety of methods are adopted for forming the layer of photocurable resin composition. For example, a method of using a solution of photocurable resin composition (coating fluid) such as dip coating, roll coating, curtain coating, etc., and a method of laminating films of a photocurable resin composition (photosensitive films) are known. Among these methods, the method of laminating photosensitive films is predominantly adopted today because it can readily and easily give a layer of photocurable resin composition having an even thickness.
Currently, in the field of printed circuit board, there is a tendency of more and more enhancing the density and precision, as a result of which a resist pattern of more improved quality is requested. Thus, a resist pattern having no pin-hole and more tightly adhering to the surface of underlying substrate is demanded. A method of laminating photosensitive films predominant today, however, is known to be unable to cope with such a demand. Since a resist pattern formed by the method of laminating photosensitive films cannot sufficiently compensate unevennesses of substrate surface which appears due to flaws from impact at the time of producing the substrate, unevenness of grinding, network of glass cloth in the substrate, bits of copper plating onto surface, etc., it is difficult to obtain a sufficient adhesion. Although this difficulty may be avoidable by laminating the films under a reduced pressure (Japanese Patent Examined Publication No. 59-3740), this requires a special and expensive apparatus.
For the above-mentioned reason, solution coating methods such as dip coating, roll coating, curtain coating and the like have become remarked again in recent years. However, these coating methods are disadvantageous in that thickness of film is difficult to control and cannot be made uniform, and pin-holes appear.
Thus, as a new method, a method of forming a photosensitive film by electrodeposition has recently been proposed (Japanese Patent Unexamined Publication No. 62-235496). This method has the following merits: (1) adhesion of resist is improved, (2) the resist well follows the unevennesses of substrate surface, (3) a photosensitive film of uniform thickness can be formed in a short period of time, and (4) since the coating fluid is an aqueous solution, the pollution of working environment can be prevented and there is no problem from the viewpoint of disaster prevention. Thus, several proposals have recently been made about the formulation of electrodepositing bath suitable for this purpose.
Electrodeposition processes can be classified into anionic processes and cationic processes. In the production of printed circuit board, anionic processes are usually adopted because of the ease of post-treatment. However, the anionic process has a problem in that copper ion eluted from copper-lined laminate board at the time of electrodeposition forms a chelate with the carboxy group of resist material which functions as a pseudo-crosslinkage, so that, after exposure to light, the unexposed area cannot be removed (developed) by the use of an alkali solution. This phenomenon is hereinafter referred to as "imperfect development (or "resist scum")".
For solving this problem, a method of adding a compound capable of forming a chelate with copper such as .beta.-diketones and acetoacetic esters (Japanese Patent Unexamined Publication No. 62-262865) and a method of adding an aminopolycarboxylic acid typified by ethylene-diamine-tetraacetic acid or salts thereof (Japanese Patent Unexamined Publication No. 61-247090) were proposed.
However, addition of these compounds could not sufficiently solve the problem, but in some cases aggravated the imperfectness of development.